Conventionally, a variety of rubber-based adhesives such as natural rubber and styrene-butadiene rubber, have been commonly used as adhesives, and acrylic resins and silicone resins have been used as materials for the production processes of electronic devices where high heat resistance is required.
Acrylic resins also have high transparency and are thus often used as optical materials for flat displays such as liquid crystal displays. However, since acrylic resins themselves are decomposed and generate volatile components when left to stand for a long time at a temperature of 200° C. or higher, especially 250° C. or higher, acrylic resins are insufficient in terms of heat resistance. Silicone resins have a wide service temperature range extending from low temperatures to high temperatures and exhibit superior heat resistance than acrylic resins. However, silicone resins also generate volatile components due to decomposition and the like when left to stand for a long time at a temperature of 250° C. or higher, especially 300° C. or higher. In addition, silicone-based adhesives have a problem in that they contain a low-molecular-weight silicone component that adversely affects electronic parts.
Examples of resins having a heat resistance of 250° C. or higher include polyimide resins. Although polyimide resins have a problem in that the handling thereof is difficult due to their high processing temperatures, it has been proposed to use a polyimide resin in the form of a siloxane-based polyimide resin copolymerized with siloxane diamine for the application as a bonding agent or an adhesive (see, for example, Japanese Patent Application Laid-Open Publication (JP-A) No. H5-200946 and JP-A No. 2004-277619). However, since the copolymerized siloxane diamine has a low molecular weight or is contained in a small amount, although these siloxane-based polyimide resins can be used as bonding agents, they are insufficient for the use as an adhesive required to have a press-bonding property at room temperature. Accordingly, polysiloxane-based polyimide resins in which a relatively high-molecular-weight polysiloxane diamine is copolymerized in a large amount have been proposed (see, for example, WO 2013/039029 and JP-A No. H9-183850).
However, even these polysiloxane-based polyimide resins are still insufficient in terms of adhesive strength and have a problem in performing uniform adhesion over a large area. It could therefore be helpful to provide a polyimide resin that generates no volatile component due to decomposition or the like even at a high temperature of 250° C. or higher and exhibits sufficiently high adhesive strength in a uniform manner over a large area when used for adhesion at room temperature; a resin composition comprising the polyimide resin; and a laminated film.